1. Field of the Invention
The present invention relates to an image forming apparatus, such as a digital copier, which reads an image on an original and forms an image, and to an image forming method.
2. Description of the Related Art
Jpn. Pat. Appln. KOKAI Publication No. 9-284479 discloses a prior-art technique relating to a digital copying apparatus that can automatically find a correction coefficient of magnification in a main scan direction. This digital copying apparatus includes memory means for storing image data that is obtained by reading two marks provided immediately after a white reference plate for shading correction, and measuring means for measuring a distance between the two marks. In this case, the digital copying apparatus reads once again a printed document on which the image data stored in the memory means is printed. The image data, which has been read once again, is stored in the memory means, and the distance between the two marks is measured. A correction coefficient for magnification adjustment is found from the first measured value and the re-measured value. Thereby, this digital copying apparatus can automatically calculate the correction value of the magnification.
In the prior-art digital copying apparatus, however, the main-scan magnification is adjusted by a machine-based mechanism. The magnification adjustment in the main scan direction is a trade-off between MTF (focusing) characteristics and magnification characteristics. Thus, there is a limit to the precision in setting the main-scan direction magnification at 100%, and an error of about ±0.5% would occur. In addition, when adjustment is executed using an image processing circuit, a very fine magnification process relating to a magnification of about 100% is executed by an image process. Consequently, moiré would possibly occur, and the application of this technique is difficult.